Origin of the magnetic anomaly and tunneling effect of europium on the ferromagnetic ordering in Eu8-xSrxGa16Ge30 (x = 0,4) type-I clathrates.

Systematic dc magnetization studies using the Banerjee criterion, Kouvel-Fisher, and magnetocaloric effect methods provide physical insights into the origin of themagnetic anomaly and the tunneling effect of europium on the ferromagnetic ordering in Eu8Ga16Ge30 type-I clathrates.We showthat Eu8Ga16Ge30 undergoes a second-order magnetic transition (SOMT) at TC ∼ 35 K, resulting from the magnetic interaction between the Eu2+ ions at the Eu2 sites, followed by a secondary magnetic transition at TL ∼10 K (indicated as amagnetic anomaly in previous studies), as a result of the magnetic interaction between the Eu2+ ions at the Eu1 and Eu2 sites. The critical exponent β = 0.388 is close to that predicted from the three-dimensional Heisenberg model (β = 0.365), while the critical exponent γ = 0.956 is close to that predicted from the mean-field model (γ = 1). The substitution of Sr2+ for Eu2+ retains the SOMT but largely reduces the transition temperatures (TC ∼ 15 K and TL ∼ 5 K), with the critical exponents β = 0.521 and γ = 0.917 close to those predicted from the mean-field model (β = 0.5 and γ = 1). These results point to the important fact that the tunneling of Eu2+ between the four equivalent sites in the tetrakaidecahedral cage tends to prevent the occurrence of a long-range ferromagnetic ordering in the type-I clathrate materials

Zintl Ions within Framework Channels: The Complex Structure and Low-Temperature Transport Properties of Na_4Ge_(13)

Single crystals of a complex Zintl compound with the composition Na_4Ge_(13) were synthesized for the first time using a high-pressure/high-temperature approach. Single-crystal diffraction of synchrotron radiation revealed a hexagonal crystal structure with P6/m space group symmetry that is composed of a three-dimensional sp^3 Ge framework punctuated by small and large channels along the crystallographic c axis. Na atoms are inside hexagonal prism-based Ge cages along the small channels, while the larger channels are occupied by layers of disordered sixfold Na rings, which are in turn filled by disordered [Ge_4]^(4–) tetrahedra. This compound is the same as “Na_(1–x)Ge_(3+z)” reported previously, but the availability of single crystals allowed for more complete structural determination with a formula unit best described as Na_4Ge_(12)(Ge_4)_(0.25). The compound is the first known example of a guest–host structure where discrete Zintl polyanions are confined inside the channels of a three-dimensional covalent framework. These features give rise to temperature-dependent disorder, as confirmed by first-principles calculations and physical properties measurements. The availability of single-crystal specimens allowed for measurement of the intrinsic low-temperature transport properties of this material and revealed its semiconductor behavior, which was corroborated by theoretical calculations

Synthesis and Physical Properties Investigations of Intermetallic Clathrates

Intermetallic clathrates have long been of interest for materials science research. The promise these materials hold for useful applications ranges from thermoelectrics to photovoltaics and optoelectronics to potentially ultra-hard materials and magnetic cooling applications. Their unique physical properties are intimately related to their intriguing structural properties. Thus a fundamental understanding of the chemistry and physics of inorganic clathrates offers the possibility to assess their potential for use in the various applications mentioned above. The purpose of the current work is to expand the current knowledge of the synthetic routes for obtaining clathrate materials, their structural, chemical, and physical properties, particularly those that from in the type I, II and VIII crystal structures. New synthesis routes are presented and used for preparation of single crystals of Na 8Si46 and Na 24Si136. Single-crystal X-ray analysis, and resistivity, Seebeck coefficient and thermal conductivity measurements are presented. In addition, two inverse clathrates with compositions Sn 24P19.3Br8 and Sn17Zn7P22Br8 have been characterized in terms of their transport properties. Since the magnetic refrigeration based on the magnetocaloric effect is a topic of great interest, type VIII Eu 8Ga16Ge30 clathrates are also explored in terms of their application for magnetic cooling

Synthesis and Physical Properties of Group 14 Intermetallic Clathrates

The search of materials relevant for thermoelectric and magnetocaloric applications, as well as materials that interact with light, is an important aspect of the materials science. Such materials can be used for solid-state power generation and refrigeration, as light sources, detectors, or controllers. Intermetallic clathrates have long been of interest for the materials science research. The promise these materials hold for useful applications ranges from thermoelectrics to photovoltaics and optoelectronics to potentially ultra-hard materials and magnetic cooling applications. Their unique physical properties are intimately related to their intriguing structural properties. Thus a fundamental understanding of the chemistry and physics of inorganic clathrates offers the possibility to assess their potential for use in the various applications mentioned above. In this work the selective, phase pure, single-crystal growth of AxSi46 and AySi136 (A = Na, K) intermetallic clathrates by the new vapor-phase intercalation method is presented. The approach appears promising for accessing regions of the equilibrium diagrams for Na-Si and K-Si clathrates that can be otherwise difficult to reach due to the greatly differing properties of the constituent elements. Physical properties of these materials were investigated in terms of single-crystal diffraction, electrical and thermal properties measurements. The synthesis and structural properties of single crystals of NaxSi136 are presented. A two-step synthetic approach was employed for the synthesis of NaxSi136 which also allowed for low temperature transport measurements of polycrystalline NaxSi136. The potential of the Eu8Ga16Ge30 type-I and VIII - EuO composites for magnetocaloric applications is discussed. The type-I clathrate - EuO composites hold promise for active magnetic refrigeration around 70 K

Synthesis of an open-framework allotrope of silicon

International audience​Silicon is ubiquitous in contemporary technology. The most stable form of ​silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies1, 2, 3, 4. Here, we report the formation of a new orthorhombic allotrope of ​silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure5; second, sodium was removed from the precursor by a thermal ‘degassing’ process. The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp3 silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties

Enhanced cryogenic magnetocaloric effect in Eu8Ga16Ge30 clathrate nanocrystals

We observe an enhanced magnetic entropy change ( DSM) at cryogenic temperatures (T<20 K) in Eu8Ga16Ge30 clathrate (type-I) nanocrystals prepared by a ball milling method. With reduction in the crystal size to 15 nm, DSM is enhanced at low temperatures, reaching the highest value ( 10 J/kg K) at 5K for a field change of 5 T. For all samples investigated, there is a cross-over temperature ( 25 K) in DSM (T) above which DSM decreases with crystal size, opposite to that observed at low temperatures. A careful analysis of the magnetic and magnetocaloric data reveals that as the crystal size decreases the magnetic interaction between Eu2þ ions on the Eu2 site governing the primary ferromagnetic transition at 35K becomes gradually weaker, in effect, altering the interaction between Eu2þ ions occupying the Eu1 and Eu2 sites responsible for the secondary ferromagnetic transition at 15 K. As a result, we have observed a strong change in magnetization and the enhancement of DSM at low temperaturePeer reviewe

Synthesis and Structural Characterization of Na_<i>x</i>Si₁₃₆ (0 < <i>x</i> ≤ 24) Single Crystals and Low-Temperature Transport of Polycrystalline Specimens

Na_<i>x</i>Si₁₃₆ clathrate-II single crystals with <i>x</i> = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na₂₄Si₁₃₆ single crystals were grown from the precursor Na₄Si₄ by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na₂₄Si₁₃₆ single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 °C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na₂₄Si₁₃₆. The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content

Synthesis and Structural Characterization of Na_<i>x</i>Si₁₃₆ (0 < <i>x</i> ≤ 24) Single Crystals and Low-Temperature Transport of Polycrystalline Specimens

Na_<i>x</i>Si₁₃₆ clathrate-II single crystals with <i>x</i> = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na₂₄Si₁₃₆ single crystals were grown from the precursor Na₄Si₄ by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na₂₄Si₁₃₆ single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 °C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na₂₄Si₁₃₆. The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content

Synthesis and Structural Characterization of Na_<i>x</i>Si₁₃₆ (0 < <i>x</i> ≤ 24) Single Crystals and Low-Temperature Transport of Polycrystalline Specimens

Na_<i>x</i>Si₁₃₆ clathrate-II single crystals with <i>x</i> = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na₂₄Si₁₃₆ single crystals were grown from the precursor Na₄Si₄ by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na₂₄Si₁₃₆ single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 °C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na₂₄Si₁₃₆. The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content